Recently, image information is handled as digital data, and, for the purpose of transmission and storage of information having high-efficiency at that time, devices that are in compliance with the MPEG (Moving Picture Experts Group phase) system or the like that performs an orthogonal transform such as a discrete cosine transform and compression using motion compensation, by using the redundancy that is unique to the image information, are widely used for both information delivery in broadcasting stations and the like and information reception in standard homes.
Particularly, the MPEG2 (ISO/IEC 13818-2) system is defined as a general-purpose image coding system and is currently used widely for a broad range of applications for the professional use and the consumer use as standards covering both an interlaced scanning image and a sequential scanning image and a standard resolution image and a high definition image. By using the MPEG2 system, for example, a code amount (bit rate) of 4 to 8 Mbps in the case of an interlaced scanning image of a standard resolution of 720×480 pixels and a code amount of 18 to 22 Mbps in the case of an interlaced scanning image of high definition of 1920×1088 pixels are allocated, whereby a high compression rate and an improved image quality can be realized.
MPEG2 is targeted for high image quality coding that is mainly suitable for broadcasting but does not respond to a coding system of a code amount (bit rate) lower than that of MPEG1, in other words, a coding system of a higher compression rate. In accordance with the popularization of mobile terminals, the request for such a coding system is predicted to increase in the future, and an MPEG4 coding system has been standardized in response thereto. Relating to the image coding system of MPEG4, a specification has been approved in December, 1998 to be an international standard as ISO/IEC 14496-2.
In addition, recently, for the purpose of image coding used for television conferences, the standardization of H.26L (ITU-T Q6/16 VCEG) is in the progress. While H.26L requires the amount of calculation according to coding and decoding that is larger than that of a conventional coding system such as MPEG2 or MPEG4, it is known that a higher coding efficiency is realized.
Furthermore, currently, as part of activities of MPEG4, the standardization of a specification, which is based on H.26L, including functions not supported in H.26L and realizing higher coding efficiency is in the process as Joint Model of Enhanced-Compression Video Coding. This standardization is internationally standardized based on the title of H.264 and MPEG-4 Part 10 (AVC (Advanced Video Coding)) in March, 2003.
In addition, the standardization of FRExt (Fidelity Range Extension) including, as extensions, a coding tool, which is required for a business, called RGB, 4:2:2 or 4:4:4 and 8×8 DCT and a quantization matrix defined in MPEG-2 has been completed in February, 2005. Accordingly, the AVC becomes a coding system capable of representing a film noise included in a movie in an improved manner as well and is a system in which it is used for a broad range of applications such as a Blu-Ray (registered trademark) Disc.
However, in these days, the request for higher-compression-rate coding required for compressing an image of about 4000×2000 pixels, which are four times those of a high vision image, and for delivering the high vision image in a limited transmission capacity environment such as the Internet has been increased. For this reason, in a VCEG (Video Coding Expert Group) under the ITU-T, reviews for improving the coding efficiency have been continuously performed.
Meanwhile, in an HEVC (High Efficiency Video Coding) system, a short-term reference picture set (hereinafter, referred to as an RPS) used for recognizing reference image specifying information that specifies a reference image in a decoding device is included in an SPS (Sequence Parameter Set) (for example, see Non-Patent Document 1).
FIG. 1 is a diagram that illustrates an example of the syntax of an RPS.
As illustrated in the second line in FIG. 1, in the RPS, inter_ref_pic_set_prediction_flag is included. Here, inter_ref_pic_set_prediction_flag is reference information that represents whether reference image specifying information that specifies a reference image of a prior image, which is an image prior to a current coding image in coding order within a GOP (Group of Picture) of the current coding image, is used as reference image specifying information of the current coding image.
Here, inter_ref_pic_set_prediction_flag is “1” in a case where it represents that the reference image specifying information specifying the reference image of the prior image is used as the reference image specifying information of the current coding image and is “0” in a case where it represents that the reference image specifying information specifying the reference image of the prior image is not used as the reference image specifying information of the current coding image.
As the third and fourth lines in FIG. 1, in a case where inter_ref_pic_set_prediction_flag is “1”, delta_idx_minus1 that is the prior image specifying information specifying the prior image is included in the RPS. More specifically, delta_idx_minus1 has a value acquired by subtracting one from a value that is acquired by subtracting the coding number of the prior image from the coding number (coding order) of the current coding image. Here, the coding number is a number that is assigned to each image within the GOP from a small value in order of coding.
In addition, as illustrated in the 13th to 23rd lines in FIG. 1, in a case where inter_ref_pic_set_prediction_flag is “0”, the reference image specifying information is included in the RPS.
FIG. 2 is a diagram that illustrates an example of inter_ref_pic_set_prediction_flag and delta_idx_minus1.
In the example illustrated in FIG. 2, the reference image specifying information of the current coding image of which the coding number is N is the same as the reference image specifying information of the prior image, of which the coding number is “N−1”, that is prior to the current coding image in coding order.
In this case, inter_ref_pic_set_prediction_flag is set to “1” that represents the reference image specifying information of the prior image is used as the reference image specifying information of the current coding image. In addition, delta_idx_minus1 is set to “0” that is acquired by subtracting “N−1” that is the coding number of the prior image from N that is the coding number of the current coding image and then, from a value of “1” that is acquired as a result of the subtraction, additionally subtracting one.